1. Field of the Invention
The present invention rlates to a process for improving the lubricity of titanium and titanium alloys. More particularly, the invention relates to a process for facilitating working of such materials, preventing seizure during cold working and producing products with excellent surface finish.
2. Statement of Related Art
In cold working of metal, use is generally made of a lubricant to impart lubricity, so as to prevent seizure, i.e., to prevent direct contact of a tool and material being worked. In the case of steel, for example, oil containing an extreme pressure agent is used in comparatively light working and a soap or solid lubricant in addition to a phosphate or oxalate is used in heavy working.
As with cold working of steel, there are various ways of working titanium and titanium alloys. More specifically, there is drawing of pipes, drawing of wires, header processing and cold rolling of sheets. Unfortunately, at the present time there is no satisfactory lubricant that is suited to the various working processes in the case of titanium and titanium alloys. In the drawing of titanium pipes, use has been made of methods in which scale formed in annealing serves as an under-film and an oily lubricant is used on top. Various resins in combination with oily film used on top of a resin film have been employed; and for the conversion coating, it is fluoride alone that has been tried.
In the case of cold rolling of titanium sheets, the liability of the material to seizure and the great work hardening that occurs mean that it is not possible to effect rolling with large diameter rolls. A Sendzimir mill is used with small diameter rolls such as is generally employed for stainless steel. Normally, a mineral oil-based neat oil or emulsion is used in a Sendzimir mill in order to prevent chattering. In header processing, as with other working processes, a variety of ways of improving lubricity have been considered but no practical method is commercially available.
Comparatively good lubricity has been displayed with fluoride films jointly used with soap lubricant on top of it. However, it is difficult to maintain long-term stable characteristics since the processing solution has an extremely short life. Furthermore, in the case of those titanium alloys which have particularly good resistance to corrosion it is not possible to form a film even if a fluoride is used.
In cold rolling of titanium or titanium alloy sheets, productivity is very poor even with small diameter rolls since seizure is likely to occur. There is no choice but to roll material and achieve a low draft of 15% or less per pass. Methods of heating to produce a thin oxide film in order to resolve such problems have been considered but they have not been found to be satisfactory. When fluoride-based conversion films (as noted above) are used, excellent effects have been achieved with respect to seizure, however, there is a serious drawback in connection with the short life of the processing solution.
An object of the present invention is to resolve these problems and provide a process for improving lubricity even when heavy working is effected in cold working of titanium or titanium alloys such that there is no seizure or breakage. A further object is to provide improved lubricity that is stable with elapse of processing time.